Key investigations

The INR measures the anticoagulant effect of warfarin. It is a good predictor of toxicity and correlates with the risk of serious bleeding.

Measure the INR in patients who have ingested more than 0.5 mg/kg of warfarin, or deliberately ingested any amount of warfarin. Repeat the INR every 6 hours (initially) to define the extent of anticoagulation. Risk of serious bleeding from warfarin poisoning based on INR gives the risk of serious bleeding based on the INR.

Other investigations for warfarin poisoning are:

  • full blood count
  • blood group and cross-match.
Table 1. Risk of serious bleeding from warfarin poisoning based on INR

INR

Risk of serious bleeding

INR less than 5

low

INR 5 to 9

moderate

INR 9 or more

high

Note: INR = international normalised ratio